Freshwater Crayish 16:7–14, 2008 Copyright ©2008 International Association of Astacology ISBN: 978-0-9581424-8-9 print / 978-0-9805452-0-3 online 7 Jouni Tulonen, 1 esa erkamo, 1 Japo Jussila 2,3,† and ari mannonen 2 1 Finnish Game and Fisheries Research Institute, Evo Fisheries Research Station, 16970 Evo, Finland 2 Crayish Innovation Center, Meijeritie 1, 17200 Vääksy, Finland. 3 Department of Bioscience, University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland † Corresponding Author — E-mail: japo.jussila@uku.i The Effects of Minimum Size Regulations and Exploitation on Population Dynamics of Noble Crayish (Astacus astacus (Linnaeus)) in a Small Lake in Central Finland: A Seven Year Study INTRODUCTION Crayish trapping has traditionally been strictly regulated not only in Finland (i.e., both minimum size and trapping season regulations) (Huner et al. 1991; Westman 1992), but elsewhere in Scandinavia and Europe where similar regulations are in place (Skurdal et al. 1986; Skurdal and Taugbøl 1992; Westman and Westman 1992; Pöckl 1999; von Lukowicz 1999; Skurdal et al. 2002; Vigneux et al. 2002). In contrast, Erkamo et al. (1992) proposed that the management strategy should be based on a liberal and lexible trapping policy, eficient exploitation of crayish populations, and even removal of undersize crayish. This strategy is based on combating crayish plague (Aphanomyces astaci Schikora), and recommends that populations should be exploited and distributed instead of spared. Management strategies have been motivated by several, sometimes even conlicting interests, such as size of commercial catch, conservation issues, recruitment, and recreation (Cerenius et al. 2002; Reynolds et al. 2002; Skurdal et al. 2002; Vigneux et al. 2002). The minimum legal size, 10 cm total length (TL), was dropped from Finnish Fishery Law in 1993 and the management of the trapping regulations in that respect were delegated to local ishing right (= licence) owners and local authorities. However, there is little data available on the possible consequences of the exploitation of all mature size crayish on wild stocks, especially under Scandinavian climactic conditions (Huner et al. 1991; Huner et al. 1992). Previously, preliminary research on this topic has been carried out mostly in North America (Momot and Gowing 1977; Morgan and Momot 1988; Momot 1991, 1993). Some other researchers have investigated the effects of long term trapping on noble crayish populations, especially the stunting phenomena (Huner and Lindqvist 1985, 1988; Skurdal et al. 1991). In addition to abiotic factors (i.e., climate) affecting crayish population dynamics, and especially production, different biotic factors can also change crayish populations, including predation pressure by humans (Pursiainen and Erkamo 1991; Lodge and Hill 1994). Exploitation has been referred to as a compensatory form of mortality (Momot 1991) to which the population then responds. Exploitation may increase the productivity of a crayish population, but also decrease the mean size of the crayish. Alternative outcomes of exploitation could be faster growth or changes in the survival and sex ratio of the population (Momot 1991), larger mean size in the catch (Huner and Lindqvist 1988), and additional reports of species-speciic differences in response to exploitation (Lodge and Hill 1994). Furthermore, adherence to strict management regulations, such as minimum size and legal trapping season, might be necessary to ensure commercially acceptable productivity in noble crayish stocks (Momot 1991; Skurdal et al. 2002). In North America, exploitation of Orconectes virilis (Hagen) has been shown to slightly increase reproduction effort and survival of juveniles, which tends to compensate for the removal of large size crayish from the population (Momot 1993). Over time, the O. virilis population matured at a smaller size, and a younger age. These responses resulted in an increased population size showing that r-selected crayish were well adapted to exploitation. Abstract.— A study was carried out over seven years to monitor the effects of different trapping strategies and intensive exploitation on the dynamics of a noble crayish (Astacus astacus) population in the small (42 ha) Lake Linnajärvi in southern Finland. Three different exploitation strategies were used in isolated parts of the lake: a minimum capture size of 9 cm TL, a minimum capture size of 10 cm TL, and a total ban on trapping (control area). The effects of different exploitation strategies were monitored by test trappings and mark-recapture. The variables tested included population size structure, proportion of reproductive females, reproduction potential, growth, sex distribution, size at maturity, and catch per unit effort (CPUE). The different experimental management strategies led to the following outcomes: reproduction declined slightly due to exploitation; changes were observed in the mean size of the catch; a decline in the market size proportion of the catch; and an increase in the proportion of small sized crayish. Growth was not affected. The conclusions were that both trapping strategies resulted in a similar population response, but it is possible that some of the responses could have been masked by climatic and other factors. [Keywords.— catch; noble crayish; population dynamics; size regulation; trapping].